In arbuscular mycorrhizal (AM) plants, the plant delivers photoassimilates to the arbuscular mycorrhizal fungus (AMF), whereas the mycosymbiont contributes, in addition to other beneficial effects, to phosphate (PO4(3-)) uptake from the soil. Thereby, the additional fungal carbon (C) sink strength in roots and improved plant PO4(3-) nutrition may influence aboveground traits. We investigated how the foliar metabolome of Plantago major is affected along with the development of root symbiosis, whether the photosynthetic performance is affected by AM, and whether these effects are mediated by improved PO4(3-) nutrition. Therefore, we studied PO4(3-)-limited and PO4(3-)-supplemented controls in comparison with mycorrhizal plants at 20, 30, and 62 days postinoculation with the AMF Rhizophagus irregularis. Foliar metabolome modifications were determined by the developmental stage of symbiosis, with changes becoming more pronounced over time. In a well-established stage of mature mutualism, about 60% of the metabolic changes and an increase in foliar CO2 assimilation were unrelated to the significantly increased foliar phosphorus (P) content. We propose a framework relating the time-dependent metabolic changes to the shifts in C costs and P benefits for the plant. Besides P-mediated effects, the strong fungal C sink activity may drive the changes in the leaf traits.